The book interleaves the career of serial killer/mutilator/rapist Joseph Vacher with the development of modern (or at least modern-like (or at least not too pseudoscientific)) forensic techniques through Lombroso, Bertillon, and most relevant to these cases, Lacassagne (Lombroso's rival).
Vacher's early career is the most interesting part. After being spurned by a woman, he bought a gun. Unbeknownst to him, it had been loaded with half powder charges. So he managed to shoot the woman four times, and himself twice in the head, and both survived. He was placed in a mental institution, but released 'fully cured' a year later. Of course that's when he went on to murder, disembowel and rape a dozen or more teenage girls and boys over the next few years. This part of the story gets rather tedious, as he settles on a successful MO, and the murders are depressingly alike, so it's good to have it interleaved with the developments of forensics, and then the trail of evidence (and hard work) that leads to his capture and trial, his failed insanity defense, and ultimate execution.